MediaElements fails to load the data in some cases.
https://bugs.webkit.org/show_bug.cgi?id=60760

WebKitWebSourceGStreamer is the interface between WebKit and GStreamer
that uses the ResourceHandle API to request data and pass it down. For
our builds it is absolutely essential that we have a NetworkingContext
available there, in order to get access to the QNetworkAccessManager.
No access means we basically cannot load the video. The WebSource gains
access to the NetworkingContext through a WebCore::Frame pointer it has.

MediaPlayerPrivateGStreamer is responsible for propagating a pointer of
the WebCore::Frame to the WebKitWebSource in
mediaPlayerPrivateSourceChangedCallback. In there we used the MediaPlayer's
frameView() accessor to access the frame. However the frameView() member
is only set through the render tree's RenderVideo, which is rather unreliable
given that some sites create "fake" video tags initially that only become
visible later (or never).

A more reliable way is to simply use the document of the MediaPlayerClient,
which is provided at constructor time.